The notion and practice of reusing or "recycling" old asphaltic concrete paving is not novel. Both have been known and employed for many decades, since, indeed, the early days of bituminous paving. But as the use of bituminous paving grew and the cost of asphalt correspondingly declined, the need for and thus the use of recycling all but disappeared. Now, however, the abrupt and rapid increases in the price of crude oil and other energy in the past few years has revived asphalt paving recycling as an economically feasible and desirable practice. Currently, there are basically three approaches.
"Surface Recycling" is one of several methods in which the surface of existing asphalt paving is planed, milled or heated in place and scarified. The material removed is then remixed, relaid and rolled. Additional new hot asphalt, softening agents, aggregates or combinations of these may be added during the remixing stage. The end product when relaid may form the final surface or may be overlaid with fresh asphalt.
"Cold-Mix Recycling" is another of several methods in which the entire pavement structure, including sometimes untreated base material, is reprocessed in place or removed and processed elsewhere. The ingredients are mixed cold and may be reused as an aggregate base material. Asphalt or other materials or both can also be added during the processing to increase the strength of the material as an aggregate base. In any event, a final surface of fresh asphalt is necessary.
"Hot-Mix Recycling" is still another of a number of methods where the major portion of the existing pavement structure, sometimes including the underlying untreated base material, is removed, sized and mixed hot with fresh asphalt at a central plant. New aggregate or a softening agent or both may be added at the same time. The end product is much more versatile and can be laid hot as an asphalt base, binder or final surface.
In the hot-mix technique, with which the present invention is concerned, the old or "aged" asphaltic concrete pavement or mix is first broken up, hauled away and then appropriately sized in a crushing-screening plant. No matter what particular method of hot-mix recycling is used, the sized, aged mix must be heated in some manner in order to reactivate the old asphalt and restore its plasticity and workability. But if its temperature is raised too far two things happen. The essential characteristics of the old asphalt are impaired or destroyed by firing or coking and large quantities of smoke are generated, resulting in plant emissions prohibited by current and increasing environmental pollution control standards.
Bearing upon the temperature and smoke problems is the nature of the plant in which recycling is achieved. In the "conventional" type plant, aggregate is first heated in a dryer and then combined with asphalt in a mixer, such as a "pugmill". When such a plant is adapted for recycling, new aggregate is heated to, say, 450.degree.-600.degree. F. in the dryer and then combined in the mixer with the aged asphalt at stockpile temperature. The latter is heated by heat transfer from the hot fresh aggregate, whereupon fresh asphalt or softening agents or both are added. The addition of virgin aggregate may be necessary to correct "gradation" problems, i.e., too many or too few fines, in the aged mix. Virgin aggregate may be also necessary because if merely fresh asphalt is added there may then be too much asphalt and the resulting mix is too "fat" or "rich", impairing its stability besides wasting asphalt. The same is true if just softening agents are added. For instance, if the aged mix is brittle owing to low residual asphalt penetration, then a greater amount of new high penetration asphalt plus some virgin aggregate are needed to obtain a final mix or proper asphalt penetration having a quality which meets current road building standards and specifications. If, on the other hand, the old asphalt is of good, usable penetration quality, but the old aggregate gradation is improper, e.g., too many or too few fines, then virgin aggregate and again proper penetration asphalt are necessary to provide a recycled mix of proper quality. Between these extremes there can, of course, be cases in which either virgin aggregate or fresh asphalt may not be required at all. In any event, though the foregoing technique avoids the firing and smoke problems, relying on the heat of the fresh aggregate only to heat the aged mix has several drawbacks. In the first place, the amount of aged mix which can be used is limited if it is to be heated sufficiently. Next, heat is wasted since it is used only to heat the fresh aggregate and thereafter is disposed of without further use. If, on the other hand, the aged mix is directly heated by being sent through the dryer together with the fresh aggregate, firing and smoke problems emerge unless burner heat is reduced or its flame shortened, extra cooling air is introduced or other expedients are used, all of which result in plant production only about 50% of normal.
In the drum-mix type of plant, in effect, the dryer itself is used both to dry the aggregate and to mix in the asphalt. The burner at one end dries and heats the aggregate which cascades through the drum while the asphalt is introduced into the aggregate at a point sufficiently remote from the burner to prevent firing and smoke. The resulting mixture is discharged from the drum ready for use. An example of a drum-mixer of this type is found in U.S. Pat. No. 3,423,422. When a drum-mixer is used for recycling, however, firing and smoke problems also occur unless steps are taken to keep down the temperature of the aged mix which is introduced into the drum, usually together with fresh aggregate. Particularly, it is vital to avoid direct contact between the burner flame and its hot combustion gases on the one hand and the aged mix on the other, especially its fines which readily incinerate. Here, too, sometimes the burner is moved back or its flame shortened or flame barriers are added in order to try to prevent the asphalt in the aged mix from igniting. Extra cooling air or even water may be introduced or the fines of the aged mix admitted further down the drum, as for example, in U.S. Pat. No. 3,999,743. All of these approaches help but do not eliminate the firing and smoke problems, at least not without impairing the final mix or the efficiency and output of the plant.
Another approach has been to build special types of drum-mixers. An example is shown in U.S. Pat. Nos. 3,845,941 and 4,000,000, in which the interior of the drum is fitted with a large number of spaced tubes running end to end through which the heat of the burner is directed, the aged mix being heated by contact with the exterior of the tubes as it tumbles through the drum. This avoids the firing and smoke problems, it is true, since the temperature of the aged mix is kept down by not being directly exposed to the burner flame. But, besides requiring a special and not inexpensive piece of equipment, the heat transfer to the aged mix is not satisfactory, at least from the standpoint of fuel consumption, and the interior of the drum easily becomes plugged between the closely spaced tubes by the aged mix. In short, despite a number of years of effort, so far as is known, no really satisfactory method or apparatus for hot mix recycling of old asphalt pavement has hitherto emerged.
Accordingly, the primary object of the present invention is to provide a method and apparatus for hot-mix recycling of old asphaltic concrete pavement which avoid the problems outlined above and, as embodied in a drum-mixer type of plant, requires but a minimum of modification and addition to the plant, both of which are relatively inexpensive and permit the plant to be readily shifted back and forth between normal and recycling operation.